In the three years Nicolas Sarkozy has been in power, his government could have been brought down many times. His iniquitous fiscal reforms, ill-advised and divisive debates such as the one on national identity, his direct meddling in media ownership and appointment of public television and radio networks, his foreign policy and decision to rejoin Nato's high command, the dismantling of public services, the disastrous reform of public hospitals – not to mention his style of government, which is more corporate than republican. These would all justify the president's departure.

A series of expenses scandals, which saw one junior minister forced to repay the €12,000 (£10,000) he billed the state for cigars and another revealed to have spent €116,500 on the hire of a private jet for an official trip to Martinique, have compounded the sense of incompetence. But little did we imagine that it was a celebrity dilettante who could bring about Sarkozy's downfall.

L'affaire Bettencourt has become l'affaire Woerth, which has itself become une affaire d'état. Liliane Bettencourt is a frail 87-year-old heiress to L'Oréal and the richest woman in France. Over the years she has given almost a billion euros to a young friend of hers, François-Marie Banier. This protege was once a beautiful young novelist and is now a high society photographer. Bettencourt's estranged daughter is suing Banier for abusing her mother's generosity.

The case has been going on for months. But it took a new turn recently when Mediapart, a paid-for news website founded by a former editor of Le Monde, Edwy Plenel, revealed interviews secretly recorded by the heiress's butler. Although the tapes were recorded illegally, they will be considered by the judges because they suggest tax evasion may have taken place and, just as importantly, reveal what looks like a suspiciously close relationship between the heiress's financial adviser and Eric Woerth, Nicolas Sarkozy's labour minister and the former treasurer of the president's party, the UMP. Woerth's wife, incidentally, used to work for the heiress as an investment adviser. She resigned two weeks ago.

But the reason Sarkozy now feels the ground trembling beneath him is that this week Mediapart revealed declarations that the heiress's former accountant made to the police. She claims that she gave envelopes stuffed with cash to many senior figures within the UMP, including Woerth and Sarkozy himself. The alleged sums include a €150,000 (£124,000) donation to the president's election campaign. Both deny any wrongdoing. But if it were proved, the gifts would be a clear breach of the law on political parties' financing.

In the national assembly, as opposition parties vented their anger, UMP MPs called for a union sacrée in the name of democracy. Union sacrée was a term coined in the first world war to describe a political truce in which the left agreed not to oppose the government. It was hardly an appropriate response to accusations of this magnitude. The Sarkozy fortress has been attacked, but the man himself has declined to comment publicly, dismissing the allegations as "libel that aims only to smear, without the slightest basis in reality".

In the meantime, Sarkozy's lieutenants are leading the charge against Mediapart and the bloodthirsty opposition in a TV and radio onslaught. I doubt that Mediapart, which sits outside Sarkozy's web of media influence, is suddenly going to fall silent.

I hope the prosecutors who opened the preliminary investigation into l'affaire Bettencourt yesterday will also keep up the pressure. The political climate in France is looking increasingly heated this summer.